The present invention relates to a machine readable hologram containing the record of parallel bar-shaped patterns, like bar code patterns, and also to methods of recording and reconstructing such a hologram. More particularly, the present invention relates to a hologram which can be reconstructed effectively with fewer restrictions on the position of a read sensor, which reads a reconstructed image of the hologram, and with a minimal lowering in the read accuracy even if the read sensor is a little out of position. Further, the present invention relates to an article including such a hologram, and also to improved hologram recording and reconstructing methods.
It is a known practice to form a bar code into a hologram and illuminate it with a laser beam for reconstruction that has a small beam diameter to thereby read the recorded bar code (for example, see Japanese Patent Application Laid-Open (KOKAI) No. 1-142784 (1989)).
However, no consideration has heretofore been given to the relationship between the direction of the bars of a bar code which is to be recorded and reconstructed and the direction of incidence of reference light at the time of recording and that of reconstructing light at the time of reading, and no attempts have been made to specify the direction of the bars and the direction of such incident light in connection with each other. Therefore, there are many restrictions on the position of a read sensor that reads a reconstructed image of the bar code. If the read sensor is not disposed in the required position, particularly in terms of the direction of propagation of the diffracted light, the bar code cannot be read accurately.